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CAPE CANAVERAL, Fla. (AP) — It was supposed to be a routine launch pad test.

But from the Apollo 1 command module at Pad 34 came a panicked voice saying, “Fire in the cockpit.”

Exactly 40 years later, the three Apollo astronauts who were killed in that flash fire were remembered yesterday for paving the way for later astronauts to be able to travel to the moon. The deaths of Virgil “Gus” Grissom, Ed White and Roger Chaffee forced NASA to take pause in its space race with the Soviet Union and make design and safety changes that were critical to the agency’s later successes.

“I can assure you if we had not had that fire and rebuilt the command module … we could not have done the Apollo program successfully,” said retired astronaut John Young, who flew in Gemini 3 with Mr. Grissom in 1965. “So we owe a lot to Gus, and Rog and Ed. They made it possible for the rest of us to do the almost impossible.”

The memorial service at the Kennedy Space Center Visitors Complex marked the start of a solemn week for NASA — today is the 21st anniversary of the Space Shuttle Challenger accident, and Thursday marks four years since the Space Shuttle Columbia disaster.

Mr. Chaffee’s widow, Martha, and Mr. White’s son, Edward III, along with NASA associate administrator Bill Gerstenmaier, laid a wreath at the base of the Space Mirror Memorial, a tall granite-finished wall engraved with the names of the Apollo 1, Challenger and Columbia astronauts and seven other astronauts killed in accidents.

Mrs. Chaffee, 69, remembered feeding her two children hot dogs for dinner that night in 1967 and knowing something was wrong when astronaut Michael Collins showed up at her home to tell her about the accident.

“My first reaction was, ‘What could have happened? He’s not flying,’ ” Mrs. Chaffee recalled before the ceremony.

NASA also hadn’t considered the countdown drill hazardous, anticipating accidents only in space. Fire rescue and medical teams were not at the launch pad. No procedures had been developed for the type of emergency the Apollo 1 crew faced.

Before his death, Mr. Grissom, the second astronaut in space, had been so disappointed with problems in the new spacecraft that at one point he hung a lemon over it, said Lowell Grissom, the astronaut’s younger brother.

After the tragedy, the command module’s hatch was changed so it opened outward, flammable materials in the cabin were replaced, wiring problems were fixed and a mixture of nitrogen and oxygen replaced the all-oxygen atmosphere.

Apollo 1’s legacy contributed to the safety culture at NASA and the successful lunar landings, said Edward White III, whose father conducted the first American spacewalk in 1965.